Plasma membrane glycoproteins and the fine-structure of uterine epithelial cell surface are two of the most likely sites of ultimate control of embryo implantation in mammals. Yet very little is known about the cell biology of the uterine epithelium, owing to the lack of an adequate in vitro system. To this end, changes will be investigated in a major class of highly negatively charged glycoproteins, the proteoglycans, and in the surface ultrastructure of mouse uterine epithelium in response to progesterone and estradiol given in vivo. This information will then be used to evaluate the response of isolated uterine epithelium to steroids in vitro, using a culture method in which isolated sheets of epithelium are maintained as sealed vesicles. The cultures will be compared in the presence and absence of endometrial stromal cells, which appear to be required for steroid action on implantation in vivo.